1. Field of the Invention
This invention relates to methods of and apparatus for prolonging the useful life of a graphite element in an optical fiber-drawing furnace. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus of such character.
2. Description of the Prior Art
An optical fiber-drawing furnace of the prior art included a graphite element of generally cylindrical shape with a reduced central cross-sectional configuration and a flange at opposite ends thereof. The element had an axial hole therethrough including a central internal element chamber for receiving a glass preform. The furnace included means for water cooling of the flanges. Means were provided for applying electric power across the flanges to resistively heat the reduced cross-sectional configuration and, hence, the central internal element chamber so that an optical fiber can be drawn from the preform. In the past, a cold graphite element was quickly heated by the rapid application of power across the flanges, which rapid application tended to create thermal shock to the graphite element. Further, in the past, there was disconcern about the temperature of the water used for cooling the furnace flanges. As a result of such disconcern, condensed atmospheric moisture on the outer furnace surface tended to enter the furnace, causing deterioration of the graphite element. After drawing optical fiber from the optical fiber-drawing furnace, at operating temperatures, it was a past practice to reduce the furnace temperature to room temperature when not in production. No mention was made, to the best of knowledge and belief by the applicant hereof, of a "bakeout procedure" in which the furnace temperature is reduced from the operating temperature to a temperature in excess of the ambient room temperature.
In the past, the typical life of the graphite element failed to exceed 15 hours of full operating temperature usage.